1. Field of the Invention
The present invention relates to a radar system for acquiring, searching, and tracking a small and fast target.
2. Description of the Related Art
A conventional radar system will be described in detail by referring to drawings. FIG. 1 is a block diagram showing a configuration of a conventional radar system. This radar system is configured by having a transmitter 1, a circulator 2, an antenna 3, a receiver 4, an AD conversion unit 5, a frequency analysis unit 6, and a target detection unit 7.
The antenna 3 converts a transmitted signal, which is transmitted from the transmitter 1 through the circulator 2, into a radio wave, and transmits the converted radio wave as a transmitted wave to a space in a designated direction. The antenna 3 receives a reflected wave generated by reflection of the transmitted wave with a target, converts it into an electric signal, and transmits the converted electric signal as a received signal to the receiver 4 through the circulator 2.
The receiver 4 performs frequency conversion of the received signal, which is transmitted from the antenna 3 through the circulator 2, and transmits it to the AD conversion unit 5. The AD conversion unit 5 converts the analog received signal, which is transmitted from the receiver 4, into a digital received signal, and transmits it to the frequency analysis unit 6. The frequency analysis unit 6 performs a frequency analysis of the digital received signal, which is transmitted from the AD conversion unit 5, and transmits a signal indicating a result of the analysis to the target detection unit 7. The target detection unit 7 detects a target based on the signal indicating the result of the analysis, which is transmitted from the frequency analysis unit 6.
To detect a small target with a radar system, there is generally known an effective method in which a pulse hit number is increased to improve a signal-to-noise ratio (SN ratio) by an integration effect of frequency analysis processing. However, in the above-described conventional radar system, in a case where the target is moving at high speed, even when the pulse hit number is set to be large, the integration effect of the frequency analysis processing cannot be obtained since a position of the target gradually moves.
In other words, in a case where movement of the target in an observation time is small, a desired integration effect can be obtained by the processing in the frequency analysis unit 6. However, as the movement of the target in the observation time becomes larger, the integration effect becomes smaller, resulting in that the desired integration effect cannot be obtained. FIG. 2 shows an integration loss in relation to the movement of the target.
It should be noted that related arts are disclosed in Japanese Patent Laid-open Applications No. 2001-166034 and No. 2000-275332.
In Japanese Patent Laid-open Application No. 2001-166034, there is disclosed a target-tracking radar system that can surely perform tracking of a target even when the target is a small and fast target. This target-tracking radar system is provided with an A/D converter which starts sampling a predetermined sampling frequency after a predetermined time has past since a transmitted pulse had started to be transmitted and which converts an analog received signal into a series of digital data, an A/D conversion control unit for controlling to shift a starting time of sampling by a sampling interval corresponding to the moving distance of the target in a case where the moving distance of the target based on target speed data and PRF data exceeds distance resolution based on the sampling frequency, and to generate the series of the digital data in the same order and position as digital data of the received pulses, and an integrator for sequentially integrating respective digital data in the same time zone which is timed since when each transmitted pulse is transmitted.
In addition, in Japanese Patent Laid-open Application No. 2000-275332, there is disclosed a radar system which has a stable pulse compression characteristic and coherent integration performance, even when the Doppler shift is caused by movement of a target, and a range bin extends between pulse hits. This radar system is provided with a plurality of Doppler correction circuits which are different in an amount of Doppler correction, and each performs pulse compression after Doppler correction in the time domain is applied to digital I and Q signals. Further, the radar system corrects respective ranges so as to be the same range bin in a case where amplitude maximum values of the ranges are different between the pulse hits. As a result of the coherent integration, the radar system selects the maximum integration value and outputs it.
As described above, in the conventional radar system, there is a problem which is that a desired integration effect cannot be obtained by frequency analysis processing in a case where a target moves at high speed, and detection probability of a target and accuracy of angle measuring are deteriorated.
In addition, in the target tracking radar system disclosed in Japanese Patent Laid-open Application No. 2001-166034, data in the same time zone is integrated since the starting time of sampling is shifted according to movement of the target. Therefore, even when the target moves at high speed, the desired integration effect can be substantially obtained. There is a problem, however, which is that a false alarm is generated when a signal with large amplitude is entered in addition to the target. Moreover, there is a problem which is that the integration effect cannot be obtained when the moving condition of the target is changed in between times.
Further, in the radar system disclosed in Japanese Patent Laid-open Application No. 2000-275332, the Doppler correction of the pulse compression processing and the range correction of the coherent integration are preformed. However, a large effect cannot be obtained in spite of a large processing load, since a decline of the SN ratio by the pulse compression processing with the Doppler frequency is extremely small in comparison with a decline of integration gain by the movement of the target. In addition, it is not practical in terms of a calculation amount since the range correction is preformed in a brute-force manner.